Effect of spinal cord transection on the endocrine and blood pressure responses to intravenous clonidine.

To determine whether the inhibitory effect of clonidine (CLON) on renin secretion is due in part to a direct action on the kidneys or due entirely to an action on the brain, the drug was administered intravenously in a dose of 30 microgram/kg to dogs in which the spinal cord had been transected in the cervical region. Renal perfusion pressure was held constant by adjusting a suprarenal aortic clamp. The decrease in plasma renin activity produced by CLON in dogs with intact spinal cords was abolished, and in 5 of 8 dogs tested, plasma renin activity rose. The decrease in blood pressure seen in control dogs was replaced by a prolonged pressor response. The ACTH response, as measured by plasma corticoids, and the growth hormone (GH) response were not significantly reduced. The data indicate that at least at this dose, the depressor response and the decrease in renin secretion produced by CLON are completely central in origin.